An "odor" is a smell. The word "odor" is not as common as "smell", and it's most often used in these phrases:

body odor

a strong odor

a pungent odor

a faint odor

a foul odor

an unpleasant odor

a strange odor

"Odors" are usually bad, although it is possible to describe an odor as "sweet" or "pleasant". For good smells, use "scent" or "fragrance".